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Copyright © 2009 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Brock Biology of Microorganisms
Twelfth Edition
Madigan / Martinko Dunlap / Clark
Overview of Viruses and Virology
Chap
ter 1
0
Lectures by Buchan & LeCleir
Copyright © 2009 Pearson Education Inc., publishing as Pearson Benjamin Cummings
I. Virus Structure and Growth
10.1 General Properties of Viruses
10.2 Nature of the Virion
10.3 The Virus Host
10.4 Quantification of Viruses
Copyright © 2009 Pearson Education Inc., publishing as Pearson Benjamin Cummings
10.1 General Properties of Viruses
Virus: genetic element that cannot replicate independently of a
living (host) cell
Virology: the study of viruses
Virus particle: extracellular form of a virus; allows virus to exist
outside host and facilitates transmission from one host cell to
another
Virion: the infectious virus particle; the nucleic acid genome
surrounded by a protein coat and, in some cases, other layers
of material
Copyright © 2009 Pearson Education Inc., publishing as Pearson Benjamin Cummings
10.1 General Properties of Viruses
Viral Genomes
Either DNA or RNA genomes
Some circular, but most linear
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Viral Genomes
Figure 10.1
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10.1 General Properties of Viruses
Viral Hosts and Taxonomy
Viruses can be classified on the basis of the hosts they
infect
Bacterial viruses (bacteriophages)
Animal viruses
Plant viruses
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10.2 Nature of the Virion
Most viruses are smaller than prokaryotic cells; range
from 0.02 to 0.3 µm
Most viral genomes are smaller than those of biological
cells
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10.2 Nature of the Virion
Viral Structure
Capsid: the protein shell that surrounds the genome of a
virus particle
Composed of a number of protein molecules arranged in
a precise and highly repetitive pattern around the nucleic
acid
Capsomer: subunit of the capsid
Smallest morphological unit visible with an electron
microscope
Copyright © 2009 Pearson Education Inc., publishing as Pearson Benjamin Cummings
10.2 Nature of the Virion
Viral Structure (cont’d)
Nucleocapsid: complete complex of nucleic acid and
protein packaged in the virion
Enveloped virus: virus that contains additional layers
around the nucleocapsid
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Comparison of Naked and Enveloped Virus Particles
Figure 10.3
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10.2 Nature of the Virion
Nucleocapsids of viruses constructed in highly symmetric ways
Helical symmetry: rod-shaped viruses (e.g., tobacco
mosaic virus)
Length of virus determined by length of nucleic acid
Width of virus determined by size and packaging of protein
subunits
Icosahedral symmetry: spherical viruses
Most efficient arrangement of subunits in a closed shell
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Icosahedral Symmetry
Figure 10.4
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Icosahedral Symmetry
Figure 10.4
Copyright © 2009 Pearson Education Inc., publishing as Pearson Benjamin Cummings
10.2 Nature of the Virion
Enveloped Viruses
Have membrane surrounding nucleocapsid; lipid bilayer
with embedded proteins
Make initial contact with host cell
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Electron Micrographs of Animal and Bacterial Viruses
Figure 10.5
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10.2 Nature of the Virion
Complex Viruses
Virions composed of several parts, each with
separate shapes and symmetries
Examples of most complex viruses in terms of structure
can be found among bacterial viruses, which contain
icosahedral heads and helical tails
Copyright © 2009 Pearson Education Inc., publishing as Pearson Benjamin Cummings
10.2 Nature of the Virion
Some virions contain enzymes critical to infection
Lysozyme
Nucleic acid polymerases
Neuraminadases: enzymes that cleave gycosidic bonds;
allows liberation of viruses from host
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10.3 The Virus Host
Viruses replicate only in certain types of cells or in whole
organisms
Bacterial viruses are typically easiest to grow; model
systems
Animal viruses (and some plant viruses) can be cultivated
in tissue or cell cultures
Plant viruses typically are most difficult because study
often requires growth of whole plant
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10.4 Quantification of Viruses
Titer: number of infectious units per volume of fluid
Plaque assay: analogous to the bacterial colony; one of
the most accurate ways to measure virus infectivity
Plaques are clear zones that develop on lawns of host
cells
Each plaque results from infection by a single virus
particle
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Quantification of Bacterial Virus by Plaque Assay
Figure 10.6
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Quantification of Bacterial Virus by Plaque Assay
Figure 10.6
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10.4 Quantification of Viruses
The number of plaque-forming units is almost always
lower than direct counts by microscopy
Inactive virions
Conditions not appropriate for infectivity
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II. Viral Replication
10.5 General Features of Virus Replication
10.6 Viral Attachment and Penetration
10.7 Production of Viral Nucleic Acid and Protein
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10.5 General Features of Virus Replication
The Phases of Viral Replication
Attachment (adsorption) of the virus to a susceptible
host cell
Entry (penetration) of the virion or its nucleic acid
Synthesis of virus nucleic acid and protein by cell
metabolism as redirected by virus
Assembly of capsids and packaging of viral genomes
into new virions (maturation)
Release of mature virions from host cell
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The Replication Cycle of a Bacterial Virus
Figure 10.8
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10.5 General Features of Virus Replication
Virus replication typically characterized by a one-step
growth curve
Latent period: eclipse + maturation
Burst size: number of virions released
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The One-Step Growth Curve of Virus Replication
Figure 10.9
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10.6 Viral Attachment and Penetration
Attachment of virion to host cell is highly specific
Requires complementary receptors on the surface of a
susceptible host and its infecting virus
Receptors on host cell carry out normal functions for cell
(e.g., uptake proteins)
Receptors include proteins, carbohydrates,
glycoproteins, lipids, lipoproteins, or complexes
Copyright © 2009 Pearson Education Inc., publishing as Pearson Benjamin Cummings
10.6 Viral Attachment and Penetration
The attachment of a virus to its host cell results in
changes to both virus and cell surface that facilitate
penetration
Permissive cell: host cell that allows the complete
replication cycle of a virus to occur
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10.6 Viral Attachment and Penetration
Bacteriophage T4: virus of E. coli; example of one of the
most complex penetration mechanisms known
Virions attach to cells via tail fibers that interact specifically
with polysaccharides on E. coli cell envelope
Tail fibers retract and tail core makes contact with E. coli
cell wall
Lysozyme-like enzyme forms small pore in peptidoglycan
Tail sheath contracts and viral DNA passes into cytoplasm
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Attachment of Bacteriophage T4 to the Cell Wall of E. coli
Figure 10.10
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10.6 Viral Attachment and Penetration
Many eukaryotes possess mechanisms to diminish viral
infections
E.g., immune defense mechanisms, RNA interference
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10.6 Viral Attachment and Penetration
Many bacteria employ restriction-modification systems
to evade viral infection
DNA destruction system; only effective against double-
stranded DNA viruses
Restriction enzymes (restriction endonucleases) cleave
DNA at specific sequences
Modification of host’s own DNA at restriction enzyme
recognition sites prevent cleavage of own DNA
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10.6 Viral Attachment and Penetration
Viral mechanisms to evade bacterial restriction systems
Chemical modification of viral DNA (glycosylation or
methylation)
Production of proteins that inhibit host cell restriction
system
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10.7 Production of Viral Nucleic Acid and Protein
David Baltimore, Howard Temin, and Renato Dulbecco
discovered retroviruses and reverse transcriptase
Shared 1975 Nobel Prize for Physiology or Medicine
Baltimore developed classification scheme for viruses
based on relationship of viral genome to its mRNA
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The Baltimore Classification System of Viruses
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10.7 Production of Viral Nucleic Acid and Protein
Once a host has been infected, new copies of the viral
genome must be made and virus-specific proteins
synthesized in order for the virus to replicate
Generation of messenger RNA (mRNA) occurs first
Typically viral genome serves as template for viral mRNA
In some RNA viruses, viral RNA itself is the mRNA
In some cases essential transcriptional enzymes are
contained in the virion
Copyright © 2009 Pearson Education Inc., publishing as Pearson Benjamin Cummings
10.7 Production of Viral Nucleic Acid and Protein
Retroviruses: animal viruses responsible for causing
certain types of cancers and acquired
immunodeficiency syndrome (AIDS)
Class VI and VII viruses
Require reverse transcriptase
Copyright © 2009 Pearson Education Inc., publishing as Pearson Benjamin Cummings
10.7 Production of Viral Nucleic Acid and Protein
Viral Proteins
Production follows synthesis of viral mRNA
Early proteins
synthesized soon after infection
necessary for replication of virus nucleic acid
typically act catalytically
synthesized in smaller amounts
Copyright © 2009 Pearson Education Inc., publishing as Pearson Benjamin Cummings
10.7 Production of Viral Nucleic Acid and Protein
Production of Viral Proteins (cont’d)
Late proteins
Synthesized later
Include proteins of virus coat
Typically structural components
Synthesized in larger amounts
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III. Viral Diversity
10.8 Overview of Bacterial Viruses
10.9 Virulent Bacteriophages and T4
10.10 Temperate Bacteriophages, Lambda, and P1
10.11 Overview of Animal Viruses
10.12 Retroviruses
Copyright © 2009 Pearson Education Inc., publishing as Pearson Benjamin Cummings
10.8 Overview of Bacterial Viruses
Bacteriophages are very diverse
Best-studied bacteriophages infect enteric bacteria
E.g., E. coli, Salmonella enterica
Most contain dsDNA genomes
Most are naked, but some possess lipid envelopes
They are structurally complex, containing heads, tails
and other components
Copyright © 2009 Pearson Education Inc., publishing as Pearson Benjamin Cummings
10.8 Overview of Bacterial Viruses
Viral Life Cycles
Virulent mode: viruses lyse host cells after infection
Temperate mode: viruses replicate their genomes in
tandem with host genome and without killing host
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10.10 Temperate Bacteriophages, Lambda, and P1
Temperate viruses: can undergo a different life cycle resulting
in a stable genetic relationship within the host
But can also kill cells through lytic cycle
Lysogeny: state where most virus genes not expressed and
virus genome (prophage) is replicated in synchrony with host
chromosome
Lysogen: a bacterium containing a prophage
Under certain conditions lysogenic viruses may revert to the
lytic pathway and begin to produce virions
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The Consequences of Infection by a Temperate Phage
Figure 10.16
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10.10 Temperate Bacteriophages, Lambda, and P1
Bacteriophage Lambda
Linear, dsDNA genome
Complementary, single-stranded regions 12 nucleotides
long at the 5′-terminus of each strand
Upon penetration, DNA ends base-pair forming the cos
site, DNA ligates and forms double-stranded circle
When lysogenic, integrates into E. coli chromosome at the
lambda attachment site (att
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Bacteriophage Lambda
Figure 10.17
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Integration of Lambda DNA into the Host
Figure 10.18
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10.11 Overview of Animal Viruses
Unlike prokaryotes, entire virion enters the animal cell
Eukaryotic cells contain a nucleus, the site of replication
for many animal viruses
Animal viruses contain all known modes of viral genome
replication
Many more kinds of enveloped animal viruses than
bacterial viruses exist
As animal viruses leave host cell, they can remove part of host
cell’s lipid bilayer for their envelope
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Diversity of Animal Viruses: DNA Viruses
Figure 10.21a
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Diversity of Animal Viruses: RNA Viruses
Figure 10.21b
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10.11 Overview of Animal Viruses
Consequences of Virus Infection in Animal Cells
Persistent infections: release of virions from host cell
does not result in cell lysis
Infected cell remains alive and continues to produce virus
indefinitely
Latent infections: delay between infection by the virus
and lytic events
Transformation: conversion of normal cell into tumor cell
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10.12 Retroviruses
Retroviruses: RNA viruses that replicate through a DNA
intermediate
Enveloped viruses
Contain a reverse transcriptase (copies information from
its RNA genome into DNA), integrase, and protease
Virion contains specific tRNA molecules
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Retrovirus Structure and Function
Figure 10.23a
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10.12 Retroviruses
Retroviruses have a unique genome
Two identical ssRNA molecules of the plus (+)
orientation
Contain specific genes
gag: encode structural proteins
pol: encode reverse transcriptase and integrase
env: encode envelope proteins
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10.12 Retroviruses
Process of Replication of a Retrovirus
Entrance into the cell
Removal of virion envelope at the membrane
Reverse transcription of one of the two RNA genomes
Integration of retroviral DNA into host genome
Transcription of retroviral DNA
Assembly and packaging of genomic RNA
Budding of enveloped virions; release from cell
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Replication Process of a Retrovirus
Figure 10.24
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IV. Subviral Entities
10.13 Defective Viruses
10.14 Viroids
10.15 Prions
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10.13 Defective Viruses
Helper viruses (defective viruses): viruses that are
parasitic on other viruses
Satellite viruses: defective viruses for which no intact
version exists; rely on unrelated viruses as helpers
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10.14 Viroids
Viroids: infectious RNA molecules that lack a protein
coat
Small, circular, ssRNA molecules
Smallest known pathogens (246–399 bp)
Cause a number of important plant diseases
Do not encode proteins; completely dependent on host-
encoded enzymes
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Viroids and Plant Disease: Healthy vs. PSTV-Infected
Figure 10.25
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10.15 Prions
Prions: infectious proteins whose extracellular form contains no nucleic acid
Known to cause disease in animals (transmissible
spongiform encephalopathies)
Host cell contains gene (PrnP) that encodes native form of
prion protein that is found in healthy animals
Prion misfolding results in neurological symptoms of
disease (e.g., resistance to proteases, insolubilty, and
aggregation)
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Mechanisms of Prion Misfolding
Figure 10.28
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10.15 Prions
Prion disease occurs by three distinct mechanisms
Infectious prion disease: pathogenic form of prion
protein is transmitted between animals or humans
Sporadic prion disease: random misfolding of a normal,
healthy prion protein in an uninfected individual
Inherited prion disease: mutation in prion gene yields a
protein that changes more often into disease-causing
form